Many times when a person reads a book there are at least several words in the book that the reader is unfamiliar with. Sometimes the meaning of these words can be gleaned by how the words are used in sentences. However, more often the reader needs to consult outside references, such as dictionaries, encyclopedias, etc. to gain an understanding of these words. Often these sources are not readily available and, thus, the reader may elect to forego such interruptions because they are time consuming and disruptive to the readers concentration.
Instead the reader may guess at the meaning of the word by its context in the sentence or paragraph. Incorrect guesses may lead to erroneous conclusions as to the meaning of the text that is being read. The frequency of misunderstanding by a reader increases with the complexity of the vocabulary used by the writer. Thus, it would be advantageous to a reader if the meaning of difficult or unusual words were made conveniently and easily accessible.